Upon arriving at ACE Chemicals, you will find that the place has been locked down and there's no responses from anyone inside. grapple to the top of the entrance of ACE Chemicals, where you'll learn the name of the guy who just attacked you; the Arkham Knight. In order to get access to the personnel still inside of the plant, you'll need to go to the security terminal down and to the left from where you're located.
The problem is that there are five armed enemies guarding it, but other than the two standing in front of it, the rest are fairly spread apart and can easily be taken down silently. For the two closest to the terminal, use a Fear Multi-Takedown from the grates. With them all gone, go up and use the terminal to find out the identities of the workers. Making sure to avoid the one platform with all of the bad guys, launch yourself to the highest point, where Batman will throw the Batscanner into the air.
The Batscanner will circle around the compound by itself, but in order to find the locations of the workers, you'll have to do something on your end. As the scanner flies around, you will see waves in the target reticule as it goes over certain points. When the waves are going crazy, press and hold X to scan that area, which will reveal a worker's location. Do this for all five and you're ready to get to rescuing them.
Something you may have noticed is that there's a lot of drones in the compound, which spells trouble for Batman as he doesn't have the Batmobile. You can quickly solve this problem by going back to the entrance and getting inside of the room with one of the workers. There are five enemies waiting for you, but they are all unarmed. Unfortunately, the worker has already been killed, but you can hit the switch right next to him to open the front doors of the plants.
Before you can just waltz right in with the "million dollar tank," you will need to create a ramp by using the power winch. Do that and boost your way into the front area, where some drones will be waiting for you, including a heavily armored one called the Diamondback. While it can shoot a few more missiles at you than the regular ones, it's nothing too dangerous and only takes a few more shots to destroy.
Once you've made scrap metal of the drones, use the winch to remove the gate nearby, then go up the ramp. You can do one of two things here, but let's rescue the worker that's close before doing anything else. Carefully navigate the Batmobile along the path to the worker, hitting the militia in the way with your suppression shots. You should see an anchor point on the door, so use the winch to destroy it, then shoot the enemies inside.
Approach the worker and some more men will drop in, including the Arkham Knight himself. Feel free to listen to the conversation a bit, but in order to continue, you'll need to access the Batmobile remote control. In doing so, you will be able to aim with Batman and then fire rounds at the enemies to knock them out. It's best to save the one closest to you for last, as hitting him will knock his body through the glass, breaking it. Whether you take everyone out by Batmobile or using your fists, rescue the worker afterwards and bring him back to Gordon outside of the facility.
To rescue the other workers, you're going to need to grapple up to that platform with all of the enemies in the first area. Two of them are armed, but the rest are unarmed, so concentrate on the more dangerous enemies first. When you're finished, access the crane controls and move the ramp to the section where you can get the Batmobile to jump off of it, because that's what we will be doing. You don't even need to rotate the ramp at all, as you will be going straight.
Once you make the jump, head into the next area, where you'll encounter more drones, with two of them being Diamondbacks. While it looks like you can go anywhere here, you'll have to do things in a pretty set order, so start by shooting up to the room with the enemies, and take them all down. After doing that, use Detective Vision to spot a weak wall, but in order to break it, you'll have to use your Explosive Gel gadget.
Switch to that and follow the prompts on the screen to place the explosive gel, then press the bottom to make it blow up, destroying the wall and revealing some toxic gas. So, in order to get rid of this gas, go back outside to the Batmobile (or use the remote) and switch to Battle Mode, then attach the winch to the pipe just below the room. Pull back until it breaks, which will quell the gas you just encountered. You'll find another worker who has been killed just beyond the gas.
There will be another weak spot here, on the floor, so do the same thing as before and drop down to find more toxic gas, a fuse box and another weak wall, with a pipe behind it. Use the fuse box first to open some vents right next to it, then blast the wall with Explosive Gel. Switch to the Batmobile and use the winch to destroy the pipe, allowing you to progress further. Another dead worker will be here, as well as another fuse box just beyond him.
This fuse box will open the elevator door right next to your location, but the elevator is broken and all the way down the shaft. Go outside via the door and get into the Batmobile, then blast the wall at the back end of the shaft to reveal an anchor point. Attach the winch and slowly move the elevator to the top, holding it in place while you exit the vehicle. Get inside of the elevator, then use the remote to move the elevator back down.
At the bottom, you'll find a total of 11 hostiles, with the final worker. All of the hostiles are unarmed, but there are some Combat Experts mixed in with them, as well as a Medic. However, you aren't able to reach the enemies from this small area, so you'll have to find a way to where they are. There's a small hole in the ceiling you can grapple to, which leads to a path with a grate covering; behind that is a spot you can drop down and even get some sneak attacks off on the enemies.
Once all of the enemies are sleeping, talk to the worker stumbling about to learn the location of Scarecrow, but you'll need to get the worker out of the plant first. Follow him to the elevator and use the Batmobile to bring it back to the surface, then exit to only have a helicopter appear. It's time to fight!
Joining the helicopter will be a few drones, one of which is a Diamondback, so make sure you take care of the drones before anything else. Unfortunately, the helicopter won't make it easy on you, as it will shoot missiles at you that are marked by red circles on the ground. Once you've destroyed the drones, you can turn your attention to the helicopter. It'll be easier to hit the helicopter with the machine gun, but it doesn't move too much to make it a hard target for the 60mm Cannon.
After you hit it a few times and cause enough damage, the helicopter will start shooting missiles of a different kind at you, with these ones being homing. You will want to shoot these with the machine gun to destroy them. The mini-boss seems to only shoot two at first, but will move on to shooting four at a time in a little bit. The final attack the helicopter will perform is a sweep across the field, shooting missiles like its normal one. Once you deal enough damage, it will explode and fall to the ground, so you can bring the worker to the cops outside of the plant.
